Car hesitating and stalling at 3000-3500 RPM

My Vauxhall Astra 1.6 SXi 52 reg hesitates and****** when RPM are over 3000, irrelevent of speed. This is mainly when motorway driving, the car will***** but not backfire and if I lift of the gas for example when coming off a junction the car will***** violently and if revs drop as in when gear down changes it will stall, no warning lights are on until engine stalls.
I have changed the spark plugs and garage have looked at it but without a warning light they are unsure of problem. I have had 2 EGR valve changes in past and it does not seem like that as hesitation is only when RPM hits 3000-3500 rather than constant. Cambelt was changed with waterpump in August 2011 and I have had new rear and middle exhaust replaced in October 2011. This happens whether engine is cold or at running temp. Help appreciated.